In LTE (Long Term Evolution) Release-10, CA (Carrier Aggregation) was introduced to realize wide-band communication beyond 20 MHz (for example, communication at 100 Hz). In CA, communication is performed by using bundled CCs (Component Carriers) under same radio base station eNB as illustrated in FIG. 6(a).
Afterwards, in LTE Release-12 and beyond, “Small Cell enhancement” was proposed, and “Inter-site CA” in which communication is performed by using bundled CCs (cells) under different radio base stations eNB has been studied for introduction as a more flexible network architecture than conventional ones (see FIG. 6(b)).
For example, as illustrated in FIG. 6(b), in a conceivable operation using “Inter-site CA” in which communication for a C-plane data requiring reliability is performed in a cell #1 (a macro cell) under a radio base station eNB#1 (a macro eNB) via a SRB (Signaling Radio Bearer), while communication for a U-plane data requiring wide-band communication is performed in a cell #10 (a small cell) under a radio base station eNB#10 (a small eNB) via a DRB (Data Radio Bearer).
In the case where the communication for the C-plane data is performed in the cell #1 under the radio base station eNB#1 via the SRB, and the communication for the U-plane data is performed in the cell #10 under the radio base station eNB#10 via the DRB as described above, the radio base station eNB#1 is expected to basically handle connections and settings between a mobile station UE and the radio base station eNB#1/the radio base station eNB#10, as illustrated in FIG. 7.
In this case, basically, it is expected that the cell #1 (the macro cell) is set as a reliable Pcell (Primary Cell), and the cell #10 is set as a supplementary Scell (Secondary Cell).
In this case, the radio base stations eNB#1 and #10 are expected to independently perform scheduling processing for the mobile station UE by using schedulers.
Here, according to the LTE specifications, the mobile station UE is expected to transmit a scheduling request to the radio base station eNB, as illustrated in FIG. 8, when uplink data to be transmitted is generated.
As illustrated in FIG. 8, when a “PUCCH (Physical Uplink Control Channel)-SR resource” that is a dedicated SR (Scheduling Request) transmission resource is assigned to the mobile station UE, the mobile station UE transmits an SR to the radio base station eNB by using the PUCCH-SR, in STEP 1.
On the other hand, the mobile station UE starts a RA (Random Access) procedure in STEP 1, when the PUCCH-SR is not assigned to the mobile station UE.
In STEP 2, the mobile station UE transmits a BSR (Buffer Status Report) to the radio base station eNB by using a PUSCH (Physical Uplink Shared Channel) corresponding to a “UL grant” assigned to the above-described SR.
Here, when the RA procedure is performed in STEP 1, the “UL grant” is designated by a “RA response” transmitted in the RA procedure.
Note that the BSR is configured to calculate and then report an amount of buffered data in each LCH (Logical Channel) for every LCG (Logical Channel Group) in which uplink data is present, as illustrated in FIG. 9.